The present invention relates generally to the field of self propelled lift trucks. More particularly, it relates to the transportable variety which require an extremely low unladen weight relative to its load carrying capacity.
There are many types of lift trucks designed for material handling applications.
A number of such lift trucks use piston-cylinder arrangements or chain-link drive mechanisms combined with sprocket wheels to raise and/or lower the lifting tines on a vertical mast. Others use an articulating linkage and gantry structure to raise and/or lower the lifting tines. While, yet others, use a telescopic lifting arm articulated about a horizontal axis perpendicular to the longitudinal axis of the lift truck to raise and/or lower the lifting tines. For example, such vehicles are disclosed in U.S. Pat. Nos. 3,858,730; 4,345,873; 4,365,921; 4,531,615; 4,621,711; 4,826,474; 4,921,075; 4,986,721; 5,061,149; 5,199,861; 5,478,192; D384,477; European Patent Specification 0 701 963 B1, and Irish Patent Specification S75661.
The materials handling lift truck of the present invention is adapted to be securely mounted and transported on the back of a carrier vehicle, such as a trailer or truck. In this manner, the lift truck can be conveniently transported directly to the work site, along with the load to be moved.
For a lift truck to be effectively used in this manner, it should be lightweight, and yet at the same time, should be capable of lifting heavy loads. Form following function dictates that to provide heavy lifting capability while minimizing the weight of the lift truck, it is advantageous for the lifting tines to have the capacity to be shifted from a position forward of the front wheels to a position behind them thereby relocating the centre of gravity of the load towards the centre of gravity of the vehicle and thus increasing the lifting capacity of the lift truck. For this reason, the aforementioned type lift truck generally has a U-shaped frame with the open end toward the front, at least two front wheels and at least one steerable rear wheel, such that the lifting device thereby extends from a most retracted position between the lateral members of the U-shaped frame.
The prior art also discloses lift trucks which utilize heavy counterweights thereby increasing the load handling capability of the lift truck. Such mechanisms are inconsistent with the need to minimize the weight of a lift truck designed to be transported on the back of a carrier vehicle, since such counterweights reduce the useful payload capability of the carrier vehicle as well as increase the moment load applied at the rear of the carrier vehicle.
Prior art lift trucks may avoid the need to use heavy counterweights by positioning the lifting tines and the load they carry between the front and the rear wheels. However, this type of design has been generally limited to applications where the load to be lifted is at ground level. For example, such vehicles are disclosed in U.S. Pat. Nos. 3,610,453 and 3,861,535.
In general, all lift trucks of the prior art, capable of being transported on carrier vehicles have the disadvantage of obstructing the field of view from the operator""s station.
The transportable lift trucks of the prior art generally have minimal ground clearance and are not as capable of travelling over rough or bumpy terrain, but are more suited to paved ground.
A feature that some lift trucks of the prior art provide is a xe2x80x9csideshiftxe2x80x9d mechanism. That is, the ability to move the tine carriage and lifting tines a fixed distance left or right of the longitudinal axis of the lift truck thus providing the vehicle operator the ability to manipulate the pickup or drop off of a load without having to readjust the position of the whole vehicle which would typically involve reversing/driving forward the vehicle in order to renegotiate a more optimum load approach. The method in which, and distance that the lifted load can be shifted from the longitudinal axis of the lift truck is critical on transportable type lift trucks since their function dictates a compact and light design.
In the case of prior art trucks that utilize an integrated pivot and xe2x80x9csideshiftxe2x80x9d assembly, namely those that utilize a telescopic lifting arm articulated about a horizontal axis as the lifting device, the lifting device thus having the capability of being displaced transversely in the horizontal plane as well as being articulated in the vertical plane, has the disadvantage, in that, with the lifting device and xe2x80x9csideshiftxe2x80x9d structure assembly being shifted transversely in the horizontal plane, there results a significant shift laterally of the lift truck""s centre of gravity; such a shift thus leading to a notable reduction in the permissible amount of xe2x80x9csideshiftxe2x80x9d in order to maintain vehicle stability. Furthermore, this arrangement has the disadvantage in that it involves high fabrication costs and significant maintenance costs in exchange for a relatively minimal guarantee of functional reliability.
In the case of prior art trucks that have a xe2x80x9csideshiftxe2x80x9d mechanism or system that connects the vertical mast to the tine carriage supporting the lifting tines, or the telescoping lifting arm to the tine carriage supporting the lifting tines; this system has the disadvantage of increasing the total space occupied by the lift truck behind the carrier vehicle, this distance corresponding to the depth of the structure required for the xe2x80x9csideshiftxe2x80x9d mechanism. This increases the length of the overall assembly being transported, including both the carrier vehicle and the lift truck being transported. This system also has the disadvantage of reducing the useful load of the carrier vehicle since the moment load applied by the lift truck is increased by an amount corresponding to this distance. The useful load of the carrier vehicle is also further reduced by an amount corresponding to the additional weight of the xe2x80x9csideshiftxe2x80x9d structure. Furthermore, the front lifting tines being separated from the lifting device by a distance corresponding to the depth of the xe2x80x9csideshiftxe2x80x9d structure has the disadvantage in that it reduces the lift truck""s useful lift capacity when in a static loading position since there is a resultant shift in the load""s centre of gravity due to this added distance being applied forward of the front wheels. The lift truck""s useful lift capacity is again further reduced by an amount corresponding to the additional weight of the xe2x80x9csideshiftxe2x80x9d structure being also applied forward of the front wheels when in a static loading position.
Another inherent disadvantage of prior art lift trucks of the type that utilize a telescopic lifting arm articulated about a horizontal axis as the primary lifting device is that the load follows a convex arc as it is being raised from the ground level to its maximum lifting height, or vice versa. This in turn tends to move the load""s centre of gravity forward thereby reducing the maximum rated lifting capacity of the lift truck. The consequence of this is apparent. Without the lift truck operator intervening and retracting the telescoping arm, the likelihood of the lift truck toppling is high.
In general, all lift trucks of the prior art, capable of being transported on carrier vehicles, in order to maintain a minimal overhang when mounted and transported on the back of a trailer or truck have the disadvantage in that the lift truck operator safety cage is intentionally kept short which results in the rear portion of the operator""s seat being outboard of or projecting past the rear of the lift truck and operator safety cage. This obviously limits the amount of protection that the back of lift truck operator safety cage can provide to the operator which consequently leaves the operator especially prone from the rear.
All lift trucks of the prior art, capable of being transported on carrier vehicles, in general, have the disadvantage in that they do not provide an automatic method for positively captivating the lift truck when mounted and transported on the back of a trailer or truck.
The object of the invention is to remedy the aforementioned disadvantages of the prior art by providing an improved self propelled lift truck, that is compact and lightweight, and is capable of being easily and quickly transported attached onto the back of a carrier vehicle.
The invention is particularly defined in the appended claims 1-13 which are incorporated into this description by reference.
The present invention provides a new and improved lift truck design that is advantageously more stable and lightweight, having an extremely low unladen weight relative to its load carrying capacity. It is compact, easily transportable, and is also simple to operate and maintain.
This invention involves a self propelled transportable type lift truck having a generally U-shaped frame formed by two spaced parallel longitudinal front members and a rear transverse member, a substantially A-shaped gantry structure having two laterally spaced upright assemblies mounted centrally and rearwardly of the U-shaped frame, at least two front wheels and at least one steerable rear wheel, such that the lifting device pivotally mounted to the A-shaped gantry structure thereby extends from a most retracted position between the longitudinal members of the U-shaped frame.
The lifting device to raise and/or lower the lifting tines comprises a telescopic lifting arm having at least one slidable section, the telescopic arm lifting device mounted between the two laterally spaced upright assemblies of the A-shaped gantry structure such that by activating a lifting cylinder, the telescopic arm lifting device is articulated about a horizontal axis (pivoting axis) substantially perpendicular to the longitudinal axis of the lift truck.
An operator""s cab is located to one side of the telescopic arm lifting device and the A-shaped gantry structure; the motive drive unit is located centrally on the rear of the lift truck""s U-shaped frame, adjacent the operator""s cab and housed beneath the telescopic arm lifting device in an area substantially between the two laterally spaced upright assemblies of the A-shaped gantry structure. When the telescopic arm lifting device is in its lowered and retracted position, as during the transport of a load, the telescopic arm lifting device is positioned directly over the motive power unit and tucked in an advantageous manner such that the field of view from the operator""s station is uninhibited.
The telescopic arm lifting device horizontal pivoting axis is centrally located aft and above both the steerable rear wheel and the motive power unit thus maximizing the counterbalance advantage of the lifting device while simultaneously improving the rear stability of the lift truck.
The present invention""s telescopic arm lifting device horizontal pivoting axis is advantageously pivotable about a vertical axis so as to vary the yaw angle of the telescopic arm lifting device, thereby providing a means for shifting laterally the tine carriage and thus effectively xe2x80x9csideshiftingxe2x80x9d the load. Furthermore, the vertical axis about which the telescopic arm lifting device pivots is approximately located centrally and rearwardly of the U-shaped frame. It will be appreciated that since the lateral shift of the lift truck""s centre of gravity is minimized by utilizing this preferred embodiment, the resultant increased stability of the lift truck enhances the operating range of the lift truck as well as the safety of the lift truck when xe2x80x9csideshiftingxe2x80x9d a load. In addition, the yaw control has, on demand, the capability to automatically self centre the telescopic arm lifting device. This simplifies the input required by the operator to centre the telescopic arm lifting device parallel to and along the lift truck""s longitudinal axis thereby again maximizing the stability and safety of the lift truck. Furthermore, it has the added benefit of simplifying the mounting process of the lift truck onto the back of a carrier vehicle, since once the telescopic arm lifting device has been automatically self-centred, no further adjustment is required of the operator to ensure that the telescopic arm lifting device is in its correct position for mounting the lift truck onto its support structure on the back of a carrier vehicle.
The invention further provides a new and advantageous telescopic arm lifting device arrangement whereby a mechanism is provided which enables an approximate vertical straight line path for the load as it is raised or lowered thereby preventing the inadvertent toppling of the lift truck when lifting its maximum rated load.
The telescopic arm lifting device supports, on its end farthest from its pivoting axis, a tine carriage capable of rotating about a horizontal axis under the action of a hydraulic actuator. The tine carriage in turn supports the lifting tines.
The tine carriage supporting the lifting tines is also automatically subjected to the action of a slave cylinder, a fluid displacement levelling system, which substantially maintains the tines"" attitude in the position from whence it began as the telescopic arm lifting device is raised or lowered.
The lift cylinder for raising and lowering the telescopic arm lifting device, and the master cylinder for controlling the action of the slave cylinder are mounted either side of the telescopic arm lifting device.
The lift truck of the present invention provides a steerable rear wheel capable of 180 degree steering for tight turning radii as well as for reducing rear wheel overhang of the lift truck while being transported on the back of a carrier vehicle.
Advantageously, the steering mechanism of the present invention that provides 180 degree steering capability of the steerable rear wheel automatically self adjusts itself to compensate for any wear and tear of the steering mechanism that may occur while operating the lift truck. Consequently, the operator of the lift truck always achieves positive and direct control of the steerable rear wheel thereby further enhancing the safe operation of the lift truck.
The steering mechanism of the present invention advantageously has a very compact and shallow profile and by virtue of its very design enables the motive power unit to be disposed in a housing substantially between the two laterally spaced upright assemblies comprising the A-shaped gantry structure, above the steerable rear wheel and below the telescopic arm lifting device thereby keeping the overall envelope height of the motive power unit housing and the telescopic arm lifting device above the steerable rear wheel to a very minimum thus enabling the operator to have an uninhibited field of view when transporting a load. Consequently, the safe operation of the lift truck is further enhanced.
The extremities of the longitudinal members of the U-shaped frame supports inclined telescoping stabilizers (outriggers), having at least one slidable section, capable of firm contact with the underlying ground surface in front of the front wheels. The outriggers stabilize the lift truck when lifting a load when the telescopic arm lifting device is extended from a most retracted position to a position forward of the front wheels.
The invention further provides a complementary means that enables the lift truck to both displace from and retract back into the rear of the carrier vehicle a support structure forming a carrier surface for the lift truck and a latching mechanism that automatically and positively captivates the lift truck to the support structure carrier surface when mounted on the rear of a carrier vehicle.
The lift truck, in accordance with the invention, is intended to be transportable on the back of a carrier vehicle and in view of applicable transportation regulations as well as concerns for safety and the dynamic effects of a trailing load, the unladen weight and the overall overhang length of the lift truck when mounted on the back of the carrier vehicle are kept to a minimum. The latter is achieved without foreshortening or compromising the length of the operator safety cage thereby still maximizing the amount of protection offered to the operator from the rear.
The lift truck design disclosed herein has the additional advantage of providing relatively large ground clearances under the U-shaped frame and the telescopic arm lifting device, as well as under the lifting tines, when transporting a load. This enables the lift truck to travel over rough and irregular terrain, as well as to negotiate street curbs or other obstacles encountered while operating in an urban environment.